1. Field of the Invention
The present invention relates to a printing position control method of an ink jet printing apparatus by which an image is printed on a printing medium while using a plurality of printing heads.
2. Description of the Related Art
In recent years, more digital copiers or printers have been rapidly used. A digital copier or printer can provide color adjustment or image processing for example and thus has been a mainstream in the field of color printing apparatuses such as a color printer or a color copier. Recent printing apparatuses use printing methods such as an electronograph method, an ink jet method, or a thermal transfer method. Among these methods, the ink jet printing method is advantageous in satisfying three factors of the price of the apparatus, the printing quality, and the running cost. Due to this reason, digital color ink jet printing apparatuses have been useful in recent years ranging from a low-cost and small apparatus such as a home printer to a high-speed and large apparatus for office application.
By the way, more digital cameras have been recently used with a diffusion rate higher than that of silver salt cameras. Thus, large-scale retailers, which conventionally have provided a service for developing silver salt photographs and a print service, recently provide a digital print service for images taken by digital cameras. Such a retailer requires a large amount of print output within a short time and thus draws a roll-like printing medium to continuously convey the medium. Then, ink is discharged from a printing head having a width corresponding to that of the printing medium to print an image. Then, after the printing of the image is completed, the printed part is cut. Thus, a continuous paper as a roll paper does not need a cut processing in the manufacture thereof and thus requires a lower cost than that of a cut sheet and can be conveyed into the apparatus by a simpler structure than that for a cut sheet. Thus, an outputted printed matter can be provided with a relatively low cost while reducing the cost of the apparatus or a frequency at which the apparatus fails. Furthermore, the use of a printing head having a width corresponding to that of a printing medium combined with the continuous paper conveying of a printing medium can provide a higher printing speed.
An ink jet printing apparatus having the structure as described above desirably minimizes factors having an influence on a printing position (e.g., dispersion, inclination, or float of a convey accuracy for conveying a printing medium). Japanese Patent Laid-Open No. 2001-277673 discloses a method for printing a predetermined test pattern to read this pattern by a previously provided imaging section to use the reading result to compensate the printing position of the printing head.
However, the conventional method as disclosed in Japanese Patent Laid-Open No. 2001-277673 has been a method that is effective when the printing position of a printing head is significantly dislocated by a distance than can be visually recognized. Thus, in a situation in recent years where a printing resolution exceeds 1000 dpi (dot/inch), the conventional method could not sufficiently cope with the dislocated printing position. Furthermore, the method described in the above patent publication is a method to compensate a steadily-caused error based on information obtained based on a previously printed test pattern. Thus, this method could not cope with a slight meandering of a printing medium for example as caused in a printing operation.
The following section will describe the structure of an ink jet printing apparatus using such a printing medium printing head used in recent years and a defect owned by such a printing apparatus.
FIG. 5 is a schematic diagram of the structure illustrating the printing section of an ink jet printing apparatus using printing medium 6 and printing heads 1 to 4. The reference numerals 1 to 4 denote printing heads for respectively jetting inks of different colors that are arranged in a convey direction (direction X). In the following description, from the paper conveying side (the right side of the drawing), the first head 1, the second head 2, the third head 3, and the fourth head 4 are provided. The reference numeral 5 denotes a platen that supports a part of the back surface of the printing medium 6 printed by the printing heads 1 to 4.
The printing medium 6 wound to have a roll-like shape at the right side of the drawing is conveyed on the platen 5 in the direction X by a convey roller 7 and a follower roller 9 by being sent between positioning sections 11 to correct the inclination of the printing medium 6. The convey roller 7 is driven by the convey motor 8 and the follower roller 9 is driven by the follower motor 10, respectively. The follower motor 10 is driven with a torque slightly smaller than that of the convey motor 8. Thus, the printing medium on the platen 5 is smoothly conveyed while being pulled in the direction X.
The convey roller 7 includes a rotary encoder 12 for measuring the rotation amount thereof. The rotary encoder 12 outputs an encoder pulse signal 101 that is inputted to a direction X timing generation circuit 21. Based on the encoder pulse signal 101, the direction X timing generation circuit 21 outputs a direction X timing signal 102. A driving control circuit 22 controls, with an appropriate timing in accordance with the inputted direction X timing signal 102 and an interval among the individual printing heads, the timings at which ink is jetted through the respective printing heads 1 to 4 (driving timing).
FIG. 6 is a timing chart for explaining examples of timings of the encoder pulse signal 101, the direction X timing signal 102, and heat signals 103 to 106 of the first to fourth heads.
The number at which the encoder pulse signal 101 is outputted while the rotary encoder 12 is rotated one time is fixed. Thus, based on the inputted pulse number, the rotation amount of the color convey roller 7 (i.e., the convey amount of the printing medium 6) can be obtained. In accordance with the number of the encoder pulse 101 which is confirmed, the direction X timing generation circuit 21 outputs the direction X timing signal 102 with a timing suitable for the printing density in the direction X.
The driving control circuit 22 transmits, while being in synchronization with the direction X timing signal 102, the respective heat signals 103 to 106 for the printing heads 1 to 4 with a timing moved by a length corresponding to an interval among which the individual printing heads are arranged. The structure as described above allows, even when the convey roller is rotated with any rotation speed, color dots to be printed on a printing medium with a fixed printing density.
By the way, even when the printing medium 6 is prevented from having a significant inclination by being sent between the positioning sections 11 as in this example, there may be a case where the printing medium 6 has a slightly meander shape at a printing section after the positioning sections 11.
FIG. 7 is a schematic view illustrating how printing is performed when such a meandering of the printing medium 6 is caused. When the printing medium 6 while being conveyed has a meander shape, a printing region of the printing medium on the platen 5 is conveyed while having an inclination as shown in FIG. 7. The inclination amount is represented by θ.
The rotary encoder 12 directly measures a rotation amount of the convey roller 7. Thus, when the printing medium 6 is inclined with the shown inclination in the convey direction, an error is caused between an interval at which the encoder pulse signal is transmitted and the convey amount in the direction X of the printing medium 6. Specifically, Vx=V×COS θ is established when assuming that a convey amount calculated based on the output of the rotary encoder is V and a practical convey amount in the direction X is Vx for example.
The driving control circuit 22 counts the direction X timing signal 102 obtained based on the encoder pulse signal 101 to generate the heat timing signals 103 to 106 of the respective printing heads. Thus, when the error as described above is included in the encoder pulse signal 101, a difference is caused among the timings at which ink is jetted through a plurality of printing heads. As a result, dots of the respective colors are printed on a printing medium at positions dislocated from one another, causing an image defect called as a color shift.
When an ink jet printing apparatus is provided as in this example so that the positioning sections 11 are provided at an appropriate position, the meandering amount can be suppressed to a certain level and can be reduced to a level that has been not problematic in the conventional structure. However, with the demand in recent years for a color image having a quality equal to that of a silver salt photograph, individual printing heads jet a small amount of ink droplets and thus significantly higher resolution is achieved by printing elements arranged in individual printing heads with a higher density and a higher printing resolution. In the circumstance as described above, even a color shift due to a printing medium having a meander shape during a printing operation is conspicuous as an image defect and is recognized as a problem that should be solved.